It is comparatively easy to perform illicit operations such as a copy and falsification on a digitalized content. Especially in a remote access, there is a need for a mechanism for preventing illicit uses intervening in a content transmission while allowing individual and home uses of a content, that is, a mechanism for a copyright protection. As a standard technique for a digital content transmission protection, there is DTCP (Digital Transmission Content Protection) developed by DTLA (Digital Transmission Licensing Administrator).
In DTCP, an inter-apparatus authentication protocol used in a content transmission and a transmission protocol of an encrypted content are defined. To make it short, the restriction includes not transmitting, by a DTCP-conforming apparatus, a compressed content that can be easily handled outside the apparatus in an unencrypted state, performing a key exchange requisite for decrypting an encrypted content according to a predetermined mutual authentication and key exchange (Authentication and Key Exchange: AKE) algorithm, and limiting a range of apparatuses that perform a key exchange by an AKE command.
DTCP has originally been defined for a content transmission on a home network using IEEE1394 as a transmission channel. Recently, movements to distribute digital contents also in homes via an IP network as represented by DLNA (Digital Living Network Alliance) are fully in progress. In this regard, DTCP-IP (DTCP mapping to IP) obtained by porting the DTCP technique to an IP network is being developed.
For example, when a commercial content accumulated in a home server, such as a broadcast content and a movie, is to be remotely used from outside, prevention of a use exceeding a personal use range based on appropriate control is desired.
In current DTCP-IP (DTCP-IP Volume 1 Specification Revision 1.4), with an intention to limit a use of a content by a third person, a remote access to a home server is limited to a terminal registered in the server. Further, in registering a terminal in a home server, a round trip time (RTT) of a command is limited to 7 milliseconds maximum, and an upper limit is set to a hop count of an IP router.
For example, there is proposed a communication system in which, while it becomes possible to share a key for a remote access by canceling the limits of the RTT and TTL in an AKE procedure in a remote access, a preregistration to a server of a terminal to be remotely accessed, a remote access usage limit of a content, and a key supply count limit are imposed so as to restrict a remote access from an unspecified number of users (see, for example, Patent Document 1).
According to the current DTCP-IP standard, however, once the registration is made in the home server of the terminal, contents in the server can be continued to be used by a remote access without having to reregister from then on. Therefore, there is a problem that once a terminal of a third person is registered in the server, that third person can continue to use the contents in the server from then on.
Patent Document 1: Japanese Patent Application Laid-open No. 2011-82952